It's old news know that The Beatles tracks are available on iTunes. Read more from The Wall Street Journal on how the deal was done.
There are still a handful of artists that have i) the creative control with their record labels, or ii) control their own masters and can determine when and how their albums will be available for download on iTunes. There are varied reasons for not getting on board with iTunes, but one of the major concerns of acts like Kid Rock and Garth Brooks is the inability of the artist/label to control album only downloads versus singles downloads.

THIS SITE IS INTENDED FOR TENNESSEE AND TEXAS AUDIENCES. ALL INFORMATION CONTAINED WITHIN THIS WEBSITE IS PROVIDED FOR INFORMATION PURPOSES ONLY AND DOES NOT CONSTITUTE LEGAL ADVICE. LICENSED BY THE SUPREME COURT OF TEXAS. NOT CERTIFIED BY THE TEXAS BOARD OF LEGAL SPECIALIZATION. LICENSED BY THE TENNESSEE BOARD OF LAW EXAMINERS. NOT CERTIFIED BY THE TENNESSEE BOARD OF LEGAL SPECIALIZATION.